GB2069794A

GB2069794A - Picture signal pre-processing method

Info

Publication number: GB2069794A
Application number: GB8100833A
Authority: GB
Original assignee: Dainippon Screen Manufacturing Co Ltd
Current assignee: Dainippon Screen Manufacturing Co Ltd
Priority date: 1980-01-21
Filing date: 1981-01-12
Publication date: 1981-08-26
Also published as: JPS56102853A; JPH0232618B2; DE3101552A1; GB2069794B; DE3101552C2; US4402015A

Description

1

SPECIFICATION

Picture signal pre-processing method This invention relates to a method for pre- 70 processing a picture signal priorto an operational circuit of a picture reproducing machine such as a color scanner and a color facsimile, and more par ticularly relates to a method for pre-processing a pic- ture signal for converting a desired reproducible density range of an original picture into an opera tional density range of the operational circuit of the picture reproducing machine.

In a picture reproducing machine such as a color scanner for plate-making, in orderto perform a pro cessing of picture signals such as a masking opera tion, a color correction operation, and so forth in an operational circuit, usually the picture signals obtained by scanning an original picture photoelec trically are logarithmically converted.

The density range of the picture signals is deter mined in advance depending on a reproducing method of a reproduction picture. For example, when the original picture is reproduced by a halftone reproduction picture, the minimum and the max- 90 imum density values correspond to halftone dot area rates of 0% and 100%, respectively, which corres pond to the minimum and the maximum values of the gradation scale required to the reproduction pic ture, and to a highlight point (the lightest point of the density range required to the reproduction picture) having a highlight density DH and a shadow point (the darkest point of the density range required to the reproduction picture) having a shadow density Ds, respectively.

Although in a usual plate-making the highlight and the shadow points correspond to the halftone dot area rates of 5% and 95%, however, they correspond to the halftone dot area rates of 0% and 100% in the following description.

The highlight density DH and the shadow density Ds depend on the original pictures, and the differ ence between the highlight density DH and the shadow density Ds or the density range depends on the original pictures.

In the picture reproducing machine, before the pic ture signals are sentto the operational circuit, usu ally the highlight density Dli and the shadow density Ds of the original picture ' are pre-processed so that the density range of the original picture may be in a certain voltage range of the operational circuit.

Such a pre-processing is important to the dicision of the density range of the reproduction picture. The controls of the highlight and the shadow densities DH and Ds of the reproduction picture to the halftone dot area rates of 0% and 100% are commonly referred to as a highlight setup and a shadow setup.

The highlight and the shadow setups also indicate the adjustment of the input conditions forthe opera tional circuit, which are different per each original picture, prior to the start of the picture reproducing machine, and thus these setups are referred to as an input setup in the following description.

In Fig. 1 there is shown a digital color scanner including a conventional pre-processor means, GB 2 069 794 A 1 wherein a signal is processed in a direction, as shown by arrows. In Fig. 2 there is shown a graph for signal conversions performed in the color scanner shown in Fig. 1.

A original picture 1 illuminated by a light source 2 is scanned photoelectrically by a scanning means (not shown). A light beam 3 through the original picture 1 is picked up by a pickup head 4 to obtain a picture signal a. Then, the picture signal a is sent to a log-converter 5 and is converted logarithmically there to output a picture density signal b which is then fed to a conventional input setup circuit 6.

The input setup circuit 6 adjusts the density range of the picture density signal b and outputs an opera- tional density signal c so that shadow and highlight values bs and bH of the picture density signal b, which correspond to the shadow and the highlight densities Ds and DH of the original picture, may correspond to minimum and maximum values c,, and cm of the operational density signal c.

The operational density signal c can be fed directly to an operational circuit of an analog color scanner.

In this embodiment, the operational density signal c is inputto an analogdigital converter 7, hereinafter referred to as A/D converter, and is converted into a digital operational density code d which is to be sent to an operational recorder 8.

The operational recorder 8 comprises an operational circuit 9 which carrys out a color correction, a masking, and other correction operations required to the color scanner, a digital-analog converter 10 which converts a digital signal into an analog signal, and a recording head 12 which records a reproduction picture onto a recording film 11 by scanning. A contact screen 13 is attached onto the front surface of the recording film 11, as occasion demands.

The adjustment of the input setup circuit 6 is carried out by allowing to coincide the density range of the operational density signal c with the input vol- tage range of the A/D converter 7 by the standard level adjustment and the span adjustment of the operational density signal c so that the shadow and the highlight values bs and bH of the picture density signal b may be coincident with the minimum and the maximum values d, and dm predetermined of the operational density code d which is output from the A/D converter 7, as follows.

For example, two different original pictures 1 and Vto be reproduced have different shadow densities Ds and Ds'and different highlight densities DH and DH', and hence the shadow values bs and bs' and the highlight values bH and bH'of the picture density signals b are different.

In orderto force the different shadow and high- light values bs, bs', bH and bH'to correspond to the minimum and the maximum values d. and dm of the operational density code d, the conversion characteristic of the setup circuit 6 are adjusted, as shown by the characteristics curves A and B for the original pictures 1 and 1' in Fig. 2, so that the shadow values bs and bs'and the highlight values bH and bH' may be converted into the minimum and the maximum operational density signals c,, and cr,,.

In the setup circuit 6 of the standard level adjustment and the span adjustment are carried out by 2 means of a level shift circuit 14 and a gain control circuit 15, respectively. That is, the level shift circuit 14 shifts the level of the operational density signal c so thatthe shadow values bs and bs'of the signals b may correspond to the minimum value c. of the operational density signal c, and the gain control circuit 15 adjusts the inclinations OA and OB of the characteristics curves A and B, i.e. the conversion gains, so thatthe highlight values b,, and b.'of the signals b may correspond to the maximum value c. of the operational density signal c, thereby setting the standard level and the span.

Meanwhile, since the picture signal a to be fed to the log-converter 5 requires a wide dynamic range such as a three decade range, i.e. a range of three figures of decimals or a density value of at least 3.0, a high stability against a direct- current drift, a gain drift, and so forth, is required to the log- converter 5. The same stability as the log-converter 5 is also required to the setup circuit 6.

Further, a good operability is also required to the setup circuit 6, for example, the adjustments can readily be performed according to the different input conditions of the different original pictures. In a conventional setup circuit 6, the operability and the stability are contrary to each other, that is, the one better, the other worse.

On the other hand, in general, each color separation picture signal of red, green or blue is processed in each color channel through a photoelectric converter, a preamplifier, a logarithmic converter, and so forth, and the deviations of the conversion characteristics orthe transfer characteristics, and the linearities of the conversion characteristics are usu- ally different per each color channel. Therefore, it is 100 necessary to correct these values of the different original pictures in each color channel in orderto be the same value so that these values corrected may directly be input to the operational circuit of the pic- ture reproducing machine.

In orderto remove the deviations of the relative characteristics among the color channels and to correctthe linearities of the conversion characteristics in each color channel, for example, it is ideal to convert the picture density signal b into a certain function by using a straight line segment function converter. However, such a straight line segment function converter having a complicated construction is unstable. Further, it is diff icuitto adjustthe straight line segment function converter.

Therefore, it is an object of the present invention to provide a method for pre-processing a picture signal prior to an operational circuit of a picture reproducing machine free from the abovementioned inconveniences, which has a high stability, a good operability, and a good convertibility.

According to the present invention there is provided a method for preprocessing a picture signal prior to an operational circuit of a picture reproduc- ing machine wherein an original picture is scanned photoelectrically to obtain the picture signal, comprising the steps of (a) reading out first conversion characteristics data stored in a memory by addressing addresses of the meory by the picture signal, and (b) changing the first conversion characteristics GB 2 069 794 A 2 data by second conversion characteristics data depending on a desired reproducible density range of the original picture.

In orderthatthe present invention may be better understood, preferred embodiments thereof will be described with, reference to the accompanying drawings, in which:

Fig. 1 is a block diagram of a digital color scanner including a conventional pre-processor means, wherein a signal is processed in a direction, as shown by arrows; Fig. 2 is a graph for signal conversions performed in the color scanner shown in Fig. 1; Fig. 3 is a block diagram of a digital color scanner including a pre-processor means according to the present invention, wherein a signal is processed in a direction, as shown by arrows; Fig. 4 is a graph for signal conversions performed in the color scanner shown in Fig. 3; Fig. 5 shows a circuit including a memory and a computer connected thereto, shown in Fig. 3; Fig. 6 shows a memory map of a table stored in the memory shown in Fig. 3; Fig. 7 is a block diagram of one embodiment of a means for rewriting a table in the memory shown in Fig. 3; Fig. 8 shows a graph of a distortion correction table having a correction curve; Fig. 9 shows a graph of a deviation value table obtained from the distortion correction table of Fig. 8; Fig. 10 shows a graph of a basic table having desired characteristics curves; Fig. 11 shows a graph of a deviation value table obtained from the basic table of Fig. 10; Fig. 12 is a block diagram of a digital color scanner including another pre-processor means according to the present invention; and Fig. 13 is a block diagram of further pre-processor means according to the present invention.

There is shown in Fig. 3 a digital color scanner including a preprocessor means which performs a method according to the present invention, wherein a picture signal is processed in a direction, as shown by arrows. The members indicated by the same numbers of Fig. 3 as thosein the conventional color scanner shown in Fig. 1 are the same as those of Fig. 1, and hence their explanations can be omitted. The color separation picture signal of each primary color of red, green or blue is processed per each color channel in the picture reproducing machine such as the color scanner. However, one of the color chan- nels will be described with reference to Fig. 3.

In Fig. 3 the analog picture density signal b is directly fed to an A/D converter 16 and is converted there into a digital picture density code e.

When the operational density code d to be input to the operational recorder 8 is expressed by eight bits, the A/D converter 16 having a resolving power cor- responding to its output signal of nine bits is used. However, as described hereinafter, no more than about the half of the input range of the A/D converter 16 is effectively utilized, and therfore its substantial resolving power is not different from that of eight bits.

3 M GB 2 069 794 A 3 The picture density code e of nime bits, which is output from the A/D converter 16, is seritto a follow ing memory 17 and addresses its addresses. Then, the memory 17 outputs an operational density code d of eight bits corresponding to the picture density code e from the address pointed out.

The operational density code cl read out of the memory 17, which is almost the same as that read out of the AID converter 7 shown in Figs. 1 and 2, is then sent to the operational recorder 8. The input range of the A/D converter 16 is sufficiently wider than the span orthe amplitude of the picture density signal b, as described above, and the span of the picture density signal b is settled in the middle of the input range.

For example, when the light beam 3 is completely shut off, the minimum value b,, corresponding to the offset level is obtained. This minimum value b. is set up somewhat higher than the zero level so that the minimum value b,) may not be below the zero level of the AID converter 16 when it is changed by the drifts, or the like.

Hence, the level of the picture density signal b is settled so that, no matter howthe minimum value b.

is changed, it may always be included in the input range of the A/D converter 16. and a minimum pic ture density code e. corresponding to the minimum value bo is utilized as a black level value.

When the strength of the light beam 3 is not decreased by the original picture at all, or the light generated from the light source 2 passes only through the carrier forthe original picture 1, the maximum value b of the picture density signal b can sufficiently come in the input range of the AID converter settled as above, and a maximum picture density code e. corresponding to the maximum value b is utilized as a white level value.

Between the black level value e. and the white level value en, the shadow and the highlight values es and eH of the picture density code e, correspond- 105 ing to the shadow and the highlight densities Ds and DH for each original picture, exist. Atable is prepared and stored in the memory 17 in advance. as shown in Fig. 4, so that minimum and maximum values d.

and dn of the operational density coded maybe read 110 out of the memory 17 by addressing the addresses of the memory 17 by the shadow and the highlight values es and eH of the picture density code e.

In Fig. 4 there are shown a characteristics line C of a solid line withinthe codespan (eH - es = ew) of the 115 picture density code e, correspond' rig to the span (bH bs) of the picture density signal along the characteristics line C there being 256 picture density codes e, and a characteristics line D of a broken line within the wider code span (eJes' = ew') of the picture density code e, corresponding to the wider span (bH' - bs') of the picture density signal b, along the characteristics line D there being more than 256 picture density codes e. There are also shown in Fig.

4 two characteristics lines Eland E2 of two-dotted lines within the same code span ew'as that of the Ii e D, wherein in order to obtain the halftone dot area rate of 50% by using the line D, the control amount + M or - M is shifted from the middle value em'of the picture density code e.

The shadow and the highlight values es and eH corresponding to the shadow and the highlight density signals bs and bH of the original picture 1 are input to a competer 18 prior to the recording of the reproduction picture, and the memory 17 is controlled by the competer 18 so thatthe table stored in the memory 17 may be rewritten depending on the shadow and the highlight values es and eH.

In Fig. 5 there is shown a circuit including the memory 17 and the competer 18 connected thereto. The picture density code e output from the A/D converter 16 is fed to the address busline 19 of the memory 17 via a bus buffer 20. The address signal is fed to the address busline 19 of the memory 17 via a bus buffer 21. The operational density code d is sent from the output busline 22 of the memory 17 to the operational recorder 8 via a bus buffer 23 and the computer 18 via a bus buffer 24.

When the memory 17 is used ordinary, the bus buffer 20 and 23 are opened. But, when the table is prepared in the memory 17 by using the computer 18, the bus buffers 21 and 24 are opened. The opening and the closing of the bus buffers 20, 21, 23 and 24 are carried out by an order signal 25 generated by the computer 18, which is sent to the bus buffers via a bus line.

The computer 18 is provided with registers 26-30 for recording the black level value e., the shadow, the middle, and the highlight values es, em and eH for the original picture 1, and the white level code em. The computer 18 is also provided with a group of registers 31 for recording conversion values el, e2,... in orderto convertthe picture density signals b into the picture density codes e forthe linearity cor- rection, a gray scale and a color patch for adjusting, other control patchs, and so forth. The computer prepares the desired table in the memory 17 according to the values recorded in the registers 26-31.

For example, when the original picture has a shadow density Ds'and a highlight density DJ, the corresponding shadow and highlight values es'and eH' are recorded in the registers 27 and 29. Then, the memory 17 and the computer 18 are connected each other via the bus buffers 21 and 24, and the computer 18 consecutively sends the address signals corresponding to the addresses between es'and eH', and also sendsthe desired data which are to be written in the addresses addressed by the address signals, to a writing input busline 32 of the memory 17 in the same time.

Since the minimum and the maximum addresses es'and eH'of the address range to be obtained correspond to the minimum and the maximum data d. and d,,,, when the codes e are in linear proportion to the codes d, the data calculation for each address of the address signal can readily by carried out according to a straight line passing through the minimum and the maximum values. The line D shown by the broken line in Fig. 4 represents the characteristics obtained, as described above.

The address number ew', which is the same as the address span and thus is hereinafter referred to as the address span, of the address range is the difference eH' - es' between the last address eJand the first addresses', and further, since the first and the 4 GB 2 069 794 A 4 last data values d. and dn corresponding to the first and the last addresses ejand eJare known, the desired characteristics exceptthe linear ones described above can also be obtained by the calcula tion.

Further, if an address and its known data are appointed in the intermediate portion of the address span ew', a characteristics line of a straight line seg ment graph can readily be obtained.

For example, in Fig. 4the middle value em'can be shifted somewhatforward or rearward from the value corresponding to the halftone dot area rate of 50%, orthe value corresponding to the halftone dot area rate of 50% can be shifted forward or rearward to the control amount M from the middle value e.', as shown bythe characteristics lines E, and E2 in Fig.

4.

in this case, the operational density code d corres ponding to the halftone dot area rate of 50% is known, and thus the middle value e.'can be 85 obtained by the following formula.

1 1 em' = e,, +em - es 2 The control amount M is predetermined in advance. Alternatively, the middle value em'is not calculated by using the above formula, but a density Dm'to be reproduced to the halftone dot area rate of 50% may be recorded in the register 28 as the middle value em' of the picture density code e in advance, and be utilized in the calculation.

On the other hand, the computer 18 can also index the desired table to be stored in the memory 17 instead of the calculation described above as follows. For example, a plurality of tables which are prepared previously depending on the deviation correction characteristics and the address spans desired, are stored in a table indexer 33 together with their index numbers, and then the computer 18 sends an index signal 34 to the table indexer 33 via a busline in orderto select the desired table. Then, the address signals 35 and the data signals 36 of the table selected are consecutively fed to the computer 18 via buslines, and the computer 18 replaces the table stored in the memory 17 with the new table fed from the table indexer 33.

In this embodiment, if all the tables desired are stored in the table indexer 33 and one of them is selected from the table indexer 33 by indexing, the table indexer33 requires a large capacity for storing the large number of tables. In orderto reduce the capacity of the table indexer 33, basic tables having the basic address span (ew = 256) and essential deviation correction characteristics are prepared in advance and are stored in the table indexer 33. The other tables having an address span of a different length from that of the basic tables can be prepared from the basic tables by extending or compressing the address span of the basic tables without changing 125 the characteristics of the basic tables.

Another table having characteristics different from those of the basic tables can be made by correcting the basic table having the closest characteristics.

Further, as shown in Fig. 13, when one more 130 memory 17'having the same functions as the memory 17 is arranged in parallel with the memory 17 prior to the operational recorder 8, while the color scanner is operated by using the memory 17, the table stored in the other memory 17' is corrected in order to meet the following original picture, and the memories 17 and 17'are alternately used. In this embodiment, even if a plurality of original pictures mounted are aligned in the direction of the cylinder's axis, the original pictures can be processed continuously without stopping the color scanner, which is very convenient and practicable.

In Figs. 6 and 7 is shown one embodiment for extending or compressing the address span ew of the basic table without changing the characteristics of the basic table.

The address span ew of the basic table is given by 256 addresses of eight bits, corresponding to 256 addresses of eight bits of the operational density code d.

The address span ew of the basic table is theoretically optional. However, when the code number and the address number are the same, the code number and the address number of a linear standard value table for obtaining the deviations, as hereinafter described, become the same, and there is no need to prepare a particular basic table, which is very convenient.

The conversion characteristics between the address number and the output code of the basic table are optional, and depend on the desired conversion characteristics, as mentioned above.

The britire address range of the memory 17 is 512 steps of nine bits and the address range of the out- put code is 256 steps of eight bits. That is, the memory 17 has a double capacity as large as the basic table. In Fig. 6 is shown the memory map of the memory 17 together with the address numbers and the output code numbers.

That is, one basic table is selected in the table indexer 33 by the index signal 34, and the address numbers and the corresponding data are sent to the computer 18 as the address signals 35 and the data signals 36. Then, the computer 18 transfers the selected basic table to the memory 17 by addressing the addresses from address 256 to address 512 in orderto store the basic table once in the memory 17, thereby obtaining a characteristics curve F shown by a solid line in Fig. 6.

Then, the basic table written in the memory 17 is rewritten in other addresses in the memory 17. For example, the data of the baSiGtable having the address span of 256 addresses are transferred in the addresses between the shadow and the highlight values es and eH without changing the conversion characteristics, thereby obtaining a characteristics curve F'shown by a broken line in Fig. 6. That is, on this occasion, the address span is extended or compressed from the address span of 256 addresses to the address span ew = eH - es. In the embodiment shown in Fig. 6, the address span ew of the characteristics curve F' is extended to (256 + n) addresses from 256 addresses of the basic table.

In Fig. 7 there is shown one embodiment of such a table rewrite means which is in practice, included in the computer 18. The address terminal A of the memory 17 is connected to a reading address signal generator 39 and a writing address signal generator 40 through bus buffers 37 and 38, either the reading or the writing address signal generator being alternately switched on.

In the reading address signal generator 39, the address number is increased one by one by a reset pulse gR which is generated by a base-ew counter41 every time that it counts the same numbers as that of 75 the address span ew of the desired table. The base-ew counter41 is reset by the reset pulse 9R.

In the writing address signal generator 40, the address number is increased one by one by a reset pulse gw which is generated by a base-256 counter 42 every 256 address counts of the basic table hav ing the address span of 256 addresses. The base-256 counter 42 is reset by the reset pulse g, In the base-ew counter 41 of an up-down counter type, the set count number % can be programmed. 85 When the set count number ew is smallerthan 256, thecounter41 is initially set to zero as the up counter, and when the set count number ew is more than 256, is initially setto the address ew as the down-counter.

The number X counted in the base-ew counter 41 is sent to a comparator 43 as a count number code. The comparator 43 detects the count number X of the base-ew counter 41 at a time when the reset pulse gw is output by the counter 42, and then compares the count number X with a comparative set value ew 2 to send a discrimination signal Yto a controller 44 for writing and reading the data into or from the memory 17. The two counters 41 and 42 are driven by a clock pulse K generated by a clock pulse generator45.

The controller 44 forthe writing and the reading determines the writing and the reading timings by the discrimination signal Y sent from the comparator 43 and the reset pulses 9R and gw of the counters 41 and 42, and outputs a writing pulse W and a reading 110 pulse R to the memory 17 at the writing and the reading timings, as hereinafter described.

The writing pulse W, when the writing instruction is given to the memory 17, opens the bus buffer 38 of the writing address signal generator 40 and closes 115 the bus buffer 37 of the reading address signal generator 39.

The reading pulse R, when the reading instruction is given to the memory 17, forces a latch circuit 46 to latch the data read out of the memory 17.

The reading of the characteristics curve F of the basic table is carried out by addressing from address 256 to address 512 one by one every time that (ew = 256 + n) number of clock pulses K are counted in the base-ew counter 41.

On this occasion, the reading pulse R is generated by the controller 44 every time thatthe reset pulse 9R is fed to the controller 44, at the timing somewhat delayed as compared with the timing that the reset pulse g,, is fed to the controller 44. The reading pulse130 GB 2 069 794 A 5 R gives the reading instruction to the memory 17 so as to read the data out of the address addressed in the memory 17, and the data read out is latched in the latch circuit 46.

The writing of the desired characteristics curve F' of the desired tableis started atthe same time as the reading of the characteristics curve F. This is per formed by addressing the addresses from the address esto the address eH one by one every time that 256 number of clock pulses K are counted in the base-256 counter 42.

In this operation, the data latched in the latch cir cuit 46 is written in the address addressed by the writing pulse W in the memory 17.

The writing pulse W is generated successively after either reset pulse gR or gw and the reading pulse R depending on the output condition, i.e. the high level of the low level, of the discrimination signal Y.

For example, when the address span of the desired table is extended, the writing pulse W is generated as follows. When the count number x of the base-ew co u nte r 41 satisf ies the fo rm u la of ew (X< -) in the comparator 43 which outputs the discrimination signal Y of the high or the low level, the controller 44 generates the writing pulse W at the timing just after the reset pulse 9R and the reading pulse R 95 are generated. When the count number X of the base-ew counter 41 satisfys the formula of (X!: ew) 2 in the comparator 43 which outputs the discrimination signal Y of the low or the high level, the controller 44 generates the writing pulse W at the timing just after the reset pulse gw is generated.

As described above, when the reading of the characteristics curve F is carried out from address 256 to address 512, the writing of the characteristics curve F' is performed from the address es to the address eH = es + ew. In this operation, in order to read outthe basic table completely, (256 + n) x 256 clock pulses are required, as described above with reference to Fig. 7, and, on this occasion, the writing of the data is carried out in 256 + n = ew addresses, with result of eH = es + (256 + n).

When the address span ew is larger than 256 addresses and n is positive, ew = 256 + n is initially set to the base-ew counter 41 and it is acted as the down-counter, as described above. In this case, the count number X in the comparator 43 is increased by n number everyone writing address increase, for example, X equals n forthe address es+l, X equals 2n forthe address es+,,.... Since the count number X is counted by the base-ew counter 41, the count number X should be smaller than ew.

The count number X is explained as follows. That is, the clock pulse number required addressing the entire addresses of the basic table is the same as that required transferring the table, and thus the first and the last addresses of the two tables are coincident along the time axis irrespective of the address span 6 GB 2 069 794 A 6 ew of the desired table.

In this case, assuming that the characteristics curve of the basic table is shown as a continuous graph, the address positions of the basic table correspond to the sampling positions of the graph, and 70 the address positions of the desired table correspond to the sampling positions of a graph whose sampling pitch is changed.

However, when the characteristic curve of the basic table is illustrated as the graph, it is actually the 75 discontinuous graph, and the data only exist in the addresses of the basic table. Hence, in orderto transferthe characteristics curve of the basic table to the addresses of the desired table, the d ata of the addresses of the basic table are transferred to the closest addresses of the desired table. Consequently, the distance between one address of the desired table and the closest address thereto of the basic table along the time axis equals the count numberX.

Then, when the address span of the desired table is compressed or reduced, the writing pulse W is generated in the similar mannerto the extended case described above. In this embodiment, the base-ew counter 41 is initially set to zero, and it is operated as the up- counter. When the count number X satisfys; the form u [a of (X, %) 2 in the comparator 43, the controller 44 generates the writing pulse W atthe timing just after the reset pulse gw is generated, and when the count number X satis35 fystheformulaof (X:5 ew) 2 in the comparator 43, the controller 44 generates the writing pulse W at the timing just after the reset pulse 9R and the reading pulse R are generated.

This operation can be applied not onlyto the adjustment of the density range forthe input setup and the deviation correction control, as described above, but to the correction of the non-linear distortion involved in each. color channel of the color scanner.

For example, when an original picture 1 of a glay scale is reproduced by a color scanner, a correction table for equalizing the reproduction characteristics between the glay scale of the original picture and the glay scale of the reproduction picture is stored in the memory. Such a correction table can be used as a non-linear distortion correction table for each color channel of the color scanner. One embodiment of such a correction table having the same addresses as the basic table is shown in Fig. 8.

In this case, the address span of the correction table prepared in the memory 17 is not restricted to 256 addresses, and the correction tables having address numbers except 256 are made to those having 256 addresses by using the address span extending orthe compressing method described above.

This non-linear distortion correction table need not to be changed every original picture, and thus is utilized in common every original picture.

The characteristics of the correction table may be recorded therein in the form of a characteristics curve G shown in Fig. 8. However, as mentioned above, in orderto use it in combination with the basictables for obtaining the basic conversion characteristics required to each original picture, its deviation values:L-G = G - H with respect to a standard linear characteristics line H are calculated every addresses, and then a deviation table is prepared from the deviation values calculated, as shown in Fig. 9..

This deviation table forthe non-linear distortion so correction requires only a small capacity for being stored, and is used in common for every original picture. The deviation table is preferably stored in a rewritable memory so that it may be corrected, as occasion demands. 85 Further, the basic tables having the characteristics curves for each original picture can conveniently be recorded in the form of the deviation tables, as shown in Figs. 10 and 11. The basic tables having characteristics' lines I and J go shown in Fig. 10 are converted into the deviation tables shown in Fig. 11 by calculating the deviation values of every address with respect to the linear standard characteristics line H.

In order to reproduce the correction talile of the curve G andthe basictablesofthe lines I andJ shown in Figs. Ef and 10, their standard characteris tics values and their deviation val, ues are added in every address.

Thus the obtained tables are rewritten in the 1 oo memory 17 according to the original pictures by using the address span extending or compressing method described above.

It is readily understood from the above description thatthe non-linear distortion involved in the color scanner can be corrected independent of the conversion characteristics curves required to each original picture, and hence the conversion characteristics curves can be obtained as ideal conversion characteristics curves excluding the distortion correction.

Therefore, the basic tables may not be stored in the table indexer 33, and, when they are obtained by calculating in the computer 18, the calculation formulae can be simplified, and further small corrections of the basic tables so as to make the desired tables can readily be done bythe computer 18.

Meanwhile, the table prepared in the memory 17 may be displayed in the form of a graph on a display means such as a color cathod ray tube (CRT), orthe like. From the graph displayed on the display means, it can be readily checked whetherthe table includes the desired correction characteristics, and while the graph is observed, the desired correction can be made to the table by means of an input means such as a key board via the computer 18.

In Fig. 12 there is shown a digital color scanner including further embodiment of a pre-processor means according to the present invention. This embodiment has the same construction as that of Fig. 3 exceptthat instead of the log-converter 5 and the A/D converter 16 shown in Fig. 3 a non-linear A/D 7 GB 2 069 794 A 7 converter 47 is used. This non-linear A/D converter 47 does not always require an exact logarithmic characteristics. That is, the non-linear AID converter 47 compresses the dynamic range of the picture density signal b input so thatthe address span ew of the picture density code e output may be included in the address range of the memory 17 according to the dynamic range.

In this case, the non-linear distortion caused by the mismatch compared with the logarithmic conversion characteristics may be removed by a non-linear correction table which is similarto the one described above in the same manner as above. This is the same as the analog log-converter.

Although the present invention has been described with reference to preferred embodiment thereof, however, various changes and modifications can be made by those skilled in the art without departing from the scope of the present invention.

Claims

1. A method for pre-processing a picture signal. prior to an operational circuit of a picture reproducing machine wherein an original picture is scanned photoelectrically to obtain the picture signal, corn- prising the steps of:

(a) reading out first conversion characteristics data stored in a memory by addressing addresses of the memory by the picture signal; and (b) changing the first conversion characteristics data by second conversion characteristics data depending on a desired reproducible density range of t he original picture.

2 A method as defined in claim 1, wherein the address span of the first conversion characteristics data curve is changed.

3. A method as defined in claim 1, wherein the conversion characteristics curve of the first conversion characteristics data is changed.

4. A method as defined in claim 2 or3, wherein a deviation correction value is contained in the conversion characteristics data.

5. A method as defined in claim 2 or3, wherein a non-linear distortion correction value is contained in the conversion characteristics data.

6. A method as defined in claim 1, wherein the first conversion characteristics data is stored in a first memory or a second memory which are alter nately used.

7. A method according to claim 1 substantially as herein before described with reference to and as illustrated in Figures 3 toll, 12 or 13 of the accompanying drawings.

Printed for Her Majesty's Stationery Office byTheTweeddage Press Ltd., Berwick-upon-Tweed, 1981. Published atthe Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.